This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. ABSTRACT Cyclophosphamide has a broad spectrum of antitumor activity and is extensively used in the treatment of pediatric solid tumors. Cyclophosphamide in combination with the topoisomerase I inhibitor, Hycamtin[unreadable] (topotecan, GlaxoSmithKline), has been evaluated in both Phase 1 and Phase 2 pediatric clinical trials. Cyclophosphamide doses were fixed at 250 mg/m2/dose for 5 consecutive days;while topotecan doses in the Phase 1 trial ranged from 0.6 to 0.75 mg/m2/dose;and were fixed at 0.75 mg/m2/dose for 5 consecutive days in the phase 2 trial. Myelosuppression was the predominant toxicity of this combination, and G-CSF support was used for neutrophil recovery in the Phase 2 trial. Responses (complete plus partial) were observed in a variety of pediatric solid tumors including rhabdomyosarcoma (67%), neuroblastoma (46%), and Ewings sarcoma (35%). Thus, as evidenced by these data, the combination of cyclophosphamide plus a topoisomerase I inhibitor appears to be active. The safety profile of Karenitecin suggests a reduced incidence of severe (NCI-CTCAE grade = 3) hematologic toxicity when compared with that of topotecan. This is of particular importance since an improved hematologic toxicity profile may reduce the need for frequent monitoring of bone marrow function and treatment interventions (for example, treatment delays, dose reductions, red blood cell [RBC] transfusions, growth factor support), thus improving patient safety, compliance, and clinical benefit. Results from 3 Phase 1 studies clearly indicate that Karenitecin can be safely administered to patients at the dose level of 1.0 mg/m2/day IV over one hour for 5 consecutive days in a 3-week treatment cycle. The principal and dose-limiting toxicity is non-cumulative, reversible myelosuppression. Gastrointestinal toxicity is generally = grade 2 and is not dose-limiting. In 4 Phase 2 studies, Karenitecin demonstrated an acceptable safety profile, moderate clinical activity in patients with malignant gliomas, and potentially significant clinical activity in patients with metastatic melanoma, ovarian and peritoneal cancer, and non-small cell lung cancer. Based on these considerations for both agents, it is medically justified to evaluate the combination of Karenitecin co-administered with cyclophosphamide, particularly given the apparent advantages of Karenitecin relative to topotecan.